In present semiconductor technology, complementary metal oxide semiconductor (CMOS) devices, such as N-type field effect transistors (NFETs) and P-type FETs (PFETs), are typically fabricated on semiconductor wafers, such as silicon wafers that have a single crystal orientation, sometimes referred to as direction. In particular, most of today's semiconductor devices are built on silicon substrates having a (100) crystal orientation.
While substrates having a (100) Si crystal orientation are preferred for NFETs due to their greater electron mobility, substrates having a (110) Si crystal orientation are preferred for PFETs due to their greater hole mobility. Typically, hole mobility values on a substrate having a (100) Si crystal orientation are roughly about 2-4 times lower than the corresponding electron mobility for the same crystal orientation. To compensate for this discrepancy, PFETs are typically designed with greater widths in order to balance pull-up currents against the NFET pull-down currents and achieve uniform circuit switching. PFETs having greater widths, however, are undesirable because they occupy a greater amount of chip area. On the other hand, hole mobilities on a substrate having a (110) Si crystal orientation are about two times higher than on a substrate having a (100) Si crystal orientation. Therefore, PFETs formed on a (110) surface will exhibit significantly higher drive currents than PFETs formed on a (100) surface. Unfortunately, electron mobilities on (110) Si surfaces are significantly degraded compared to (100) Si surfaces.
As can be deduced from the above, the (110) Si surface is optimal for PFET devices because of excellent hole mobility, yet such a crystal orientation is inappropriate for NFET devices. Instead, the (100) Si surface is optimal for NFET. Similarly, the (100) Si surface is optimal for NFET devices because of greater electron mobility, yet such a crystal orientation is inappropriate for PFET devices. What is needed, therefore, is hybrid substrates having different crystal orientations.